Well abandonment is an inevitable stage in the lifespan of a well. As oilfields world-wide continue to shift from asset to liability, wells must be dismantled carefully to minimize environmental threats and meet all regulatory requirements.
In oilfield jargon, the phrase “plug and abandon” refers to preparing a well to be closed permanently. The earliest oil wells were abandoned without any plugging, but the first plugging requirements were enacted by Pennsylvania in the 1890s. However, prior to modern regulations set in the '50s, many wells were abandoned with plugs consisting of brush, wood, paper sacks, linen or any other material that could be pushed into a well to form a basis for the dumping of one or two sacks of cement to “plug” the well. Current procedures are significantly more disciplined however.
Plugging and abandonment (P&A) regulations vary to some degree among states, but all state regulations prescribe the depth intervals that must be cemented, as well as the materials that are allowed in plugging practices. FIG. 1 provides a comparison of the plugging requirements in different states with a focus on the key elements of plugging the oil and gas strata, plugging the fresh water zone, and surface casing plugging.
Most states require that cement plugs be placed and tested across any open hydrocarbon-bearing formations, across all casing shoes, across freshwater aquifers, and perhaps several other areas near the surface, including the top 20 to 50 ft [6 to 15 m] of the wellbore. The well designer may choose to set bridge plugs in conjunction with cement slurries to ensure that higher density cement does not fall in the wellbore. In that case, the bridge plug would be set and cement pumped on top of the plug through drillpipe, and then the drillpipe withdrawn before the slurry thickened. FIG. 2A shows a well before and after a typical P&A operation.
It is also possible to P&A only part of a well. One cost effective way to enhance production is to permanently abandon the bottom of the well, but use the existing slot to sidetrack the well to reach new payzones. The cost can often be cut in half when sidetracking an existing well instead of drilling a new horizontal well. This process is known as “slot recovery.” FIG. 2B shows an example of a zone abandonment, where only the bottom of a well is plugged.
The plugging process usually requires a workover rig and cement pumped into the wellbore and can take two days to a week, depending on the number of plugs to be set in the well. The P&A work takes considerable capital to complete and provides zero return on the investment for the oil companies. Indeed, the cost to P&A a well can vary by many millions of dollars depending on location, and whether the well is offshore or onshore. Thus, most wells are plugged at the lowest cost possible following the minimum requirements set forth by regulatory bodies.
As older oil and gas fields are re-entered to exploit bypassed reserves or to develop reserves deemed uneconomical in the past, the abandoned wells within the field can present potential problems. As these old fields are reentered to apply enhanced oil recovery (EOR) technologies, such as steam, solvent, or CO2 flooding, the reservoir pressure is again increased due to the injection of these EOR fluids. When this higher pressure reaches unplugged or poorly plugged wells, there is a chance that the formation fluids will bypass the plugging materials and migrate uphole. This can cause problems with the fresh water aquifers in the area by allowing gas, oil or salt water to contaminate the fresh water. Furthermore, the oldest pre-regulation wells were never plugged and many old wells are poorly documented, their location and plugging status uncertain.
P&A responsibility does not end with the P&A activities either, nor does it end with sale of the property. If an abandoned well leaks, the operator will be held responsible for damage and to repair and remediate the well and the site. Thus, P&A activities are very important, even if P&A operations present only costs, not income.
In recognition of its strength and low permeability, cement has been typically used to create a seal between formations or to seal off the surface of the wellbore. Other materials which do not offer the same strength or durability as cement, including drilling mud, gel, and clay, can be used to fill in the spaces between cement plugs. Additionally, many states allow the use of mechanical bridge plugs in lieu of a large cement plug since the bridge plug is extremely strong and nearly completely impermeable. However, mechanical plugs are susceptible to corrosion, and therefore the regulations typically require the bridge plugs to be capped by a specified amount of cement.
The effectiveness of a permanent abandonment plug is measured by its ability to bridge the wellbore cross section, both vertically and horizontally, including all annuli, with a plugging medium that can withstand the harsh environment to which it is exposed. Indeed, regulators are increasingly demanding that operators remove sections of casing so that a cement plug may be set that is continuous across the entire borehole in a configuration often referred to as “rock-to-rock.”
Because cement must go all the way to the formation, the typical procedure is to mill out the casing and remove tons of swarf before spotting the cement. In addition to being expensive and time-consuming, this operation prevents re-entry into the wellbore and the swarf is highly abrasive to all equipment.
A new method was recently introduced that perforates uncemented casing, washes the annular space, and then mechanically places the cement across the wellbore cross section, sometimes in a single run. Although an improvement on milling, this perforate, wash, and cement (PWC) system, it can be tricky to fill all of the gaps because of the blind spots between perforations.
Thus, what is needed in the art are better methods for P&A that create a reliable barrier, are cost effective, and are safe, fast and easy to perform. The ideal method would also be easier to reverse.